The general approach of adding touch-sensing capability to an organic light emitting diode (OLED) display panel is to supplement a transparent touch sensor layer, typically made from indium tin oxide (ITO) on top of the OLED display panel. There are several possible implementations for embedding the touch sensor layer into an OLED panel.
FIG. 1 shows an on-cell touch sensor arrangement, where a touch sensor 110, in the form of a transparent conductive layer, is put on top of a substrate glass in an OLED panel 100. The on-cell touch sensor arrangement has the touch sensor 110 isolated from anode electrodes 130 by a thick layer of lower substrate glass 120 (normally, 0.4˜0.7 mm). Hence, the capacitive coupling between the anode electrodes 130 and the touch sensors 110 is relatively low. This relatively low coupling is ideal for the touch sensor 110 to sense an approaching finger. On the other hand, a display driver for driving the OLED panel 100 is located on an inner side 121 of the substrate glass 120. Such arrangement is called a chip on glass (COG) arrangement. A touch controller for sensing a signal from the touch sensor 110 needs to communicate with the display driver as well as to connect to the touch sensor 110. Hence, a chip on film (COF) arrangement to bridge the touch sensor 110 with the touch controller is required.
FIG. 2 shows an in-cell touch sensor arrangement, where a touch sensor 210, in the form of a transparent conductive layer, is put between an upper cover glass 220 and a lower substrate glass 230 in an OLED panel 200. The touch sensor 210 is isolated from anode electrodes 240 by a very thin layer of insulator 250 (˜0.5 μm). Hence, the capacitive coupling between the anode electrodes 240 and the touch sensor 210 is high. This parasitic capacitance is much greater than an induced capacitance coming from an approaching finger, lowering the sensing sensitivity and limiting the dynamic range of the touch sensor 210. On the other hand, the integration of the display driver and the touch controller, both functioning on the same integrated circuit (IC), is feasible since the IC, the touch sensor 210 and the anode electrodes 240 are all located on an inner side 231 of the lower substrate glass 230.
FIG. 3 depicts another approach simplified from the in-cell touch sensor arrangement of FIG. 2. In an OLED panel 300, anode electrodes 320 are also used as touch sensors 310. In this arrangement, display driving and touch sensing are done in time multiplexing manner. That is, at a time instant, the OLED panel 300 is either in a display driving mode or in a touch sensing mode but not both. In a typical application, the display driving mode takes up about 90% of the time while touch sensing takes up around 10% of the time only. For a PMOLED array, the frame refresh rate is usually around 100 Hz. Similar to the in-cell touch arrangement, the anode electrodes 320 and cathode electrodes 330, both in layer forms, are in close proximity of each other. Since the OLED stack layer 340 is only about 1 μm thick, the capacitive coupling between the anode 320 and the cathode 330 is still quite high. This parasitic capacitance is much greater than the induced capacitance coming from an approaching finger. Also similar to the in-cell touch arrangement, the display driver and touch controller integration is feasible and, in fact, simpler than the integration used for the in-cell sensor arrangement. However, one drawback of this simplified approach is that the unidirectional and fixed form factor nature of the anode electrodes limits the touch sensor grid to one dimension only. For applications requiring two dimensional sensing, the shared anode electrode-sensor arrangement is not feasible.
The resolution of display screen increases drastically in recent times, and consequently the number of anode electrode increases. Obviously, the touch sensing resolution for the majority of applications that are designed for finger touch sensing needs not be as high as that of the display resolution. To save cost and space, multiple anode electrodes are grouped and shorted together for touch sensing operations. Touch detection is then determined based on comparison of the aggregated or summed sense signals from the anode electrode groups. Conventionally, the anode electrodes groupings are fixed during manufacturing. This, however, poses significant challenges when a touch falls on the border of an anode electrode group, overlaps two anode electrode groups, or moves across multiple anode electrode group in a sliding or transitional movement as the aggregated or summed sense signals from the anode electrode groups suffer low signal-to-noise (SNR) ratio and false detections. Therefore, there is an unmet need to increase the touch sensing sensitivity and accuracy of the simplified in-cell touch sensor arrangement of FIG. 3 while maintaining the simplicity in integrating the display driver and the touch controller.